Display backplanes are an important component in a display device, such as a liquid crystal display (LCD), organic light emitting diode (OLED) display, or other display technology. Display backplanes include a substrate providing a platform upon which circuitry is created to cause a display screen to display images. Typically, the backplane includes an array of pixel transistors that provide an electrical signal to an array of pixel elements, such as OLED cells, to cause the picture elements or pixels to produce light that results in the image to be viewed. Additional circuitry includes row and column drivers and is typically located separately from the backplane. The row and column drivers decode the incoming video data to individually activate the pixel transistors and thereby individually control the pixels.
Because the pixel transistors in the typical case are located on the backplane itself, the pixel transistors are formed as thin film transistors (TFFs) and thereby allow for a very thin display screen such as for thin screen computer and television monitors, telephones, and other compact devices. Because the row and column drivers in the typical case are not located on the backplane, they are not necessarily TF Is. However, the row and column drivers occupy separate space, such as on an integrated circuit chip installed on a display circuit board.
The interconnections between the row and column drivers and the backplane array can be complex. As the number of rows and columns increase, the interconnect density increases. Even when the row and column drivers are silicon chips bonded to the glass, the level of interconnect complexity can become prohibitive.
It is desirable in some display screen applications to eliminate or dedicate for other purposes the space required for the row and column driver chip and/or to bring the row and column drivers into closer proximity with the pixel transistors. Therefore, it is desirable to move the row and column drivers directly onto the backplane along with the pixel transistors. However, the row and column drivers must have very fast switching capabilities whereby conventional TFT construction utilizing low mobility semiconductor channels such as amorphous silicon becomes problematic.
It is advantageous, particularly for an OLED based display, to have TFTs including a semiconductor with as large an electron mobility as possible. In general, the electron mobility directly affects transistor speed and/or transistor size. Semiconductors like amorphous silicon have field effect mobilities on the order of 0.5 cm2/V-sec. Materials such as polysilicon have higher mobilities (greater than 20 cm2/V-sec) but require higher processing temperatures and more complex fabrication procedures.